This invention relates to an exhaust gas recirculation apparatus of an internal combustion engine equipped with a supercharger such as a Diesel engine equipped with a supercharger.
Among counter-measures for exhaust gases of internal combustion engines such as a Diesel engine, exhaust gas recirculation (hereinafter referred to as "EGR") for restricting the formation of NOx by recirculating a part of the exhaust gas as an inert gas to an intake and lowering a combustion temperature so as to reduce the emission quantity of NOx in the exhaust gas has been known as effective and has therefore been put widely to practical application.
Japanese utility model application Kokai publication No. 6-40343 proposes an exhaust gas recirculation apparatus shown in FIG. 5 of the accompanying drawings for an internal engine equipped with a supercharger for effecting such an exhaust gas recirculation. This exhaust gas apparatus includes an exhaust gas recirculation pipe 1 for connecting an exhaust manifold 14 to an intake pipe 11 of an engine 13, and a reed valve 40 disposed at the connection portion between the exhaust gas recirculation pipe 1 and the intake manifold 12 as shown in FIG. 7 so that this reed valve 40 can close and open an open portion 5 formed at the connection portion.
In this exhaust gas recirculation apparatus, the EGR is effected only when the number of revolution of the engine and its load fall within a certain range. However, when the EGR is effected in a range where a mean boost pressure is higher than a mean exhaust pressure, the reed valve 40 is opened so as to reliably execute the EGR when the pulsating exhaust pressure Pe is greater than the boost pressure Pb (oblique line portion) as represented by a pressure Pv-crank angle CA graph in FIG. 6. The reed valve 40 is closed at other times so as to prevent backflow from the intake system into the exhaust gas recirculation pipe 1 and thus to reduce NOx and to prevent the drop of engine performance.
In the exhaust gas recirculation apparatus described above, when the operation is completed while the EGR operation state is switched to the non-operation state, the engine and the exhaust gas recirculation pipe radiate heat after the stop of the operation and the temperature drops. Therefore, the moisture contained in a combustion gas remaining inside the exhaust gas recirculation pipe between the EGR valve and the reed valve dews.
Soot in the combustion gas that adheres to the inner surface of the exhaust gas recirculation pipe dissolves in this dewing water and generates strongly acidic water containing sulfuric acid ions and nitric acid ions. Because this sulfuric acid remarkably corrodes the exhaust gas recirculation pipe, there remains the problem that engine life is shortened.